The present invention relates to the fabrication and further modification of material nano-structures, which have great potential in field emission applications.
Since the discovery of cone-like structures on an ion bombarded glow discharge cathode by Guentherschulze and Tollmien (Z. physik 119, p.685, 1942), surface texturing of various materials has aroused great interests. One of the most important applications of the textured surfaces is related to their field emission related properties. Arrays of cones or pyramids have been successfully used in field desorption mass spectroscopy (Beckey et. al, J. Phys. E. 12, p72, 1979). They also have potential to be used as the electron source of ultrahigh vacuum gauges and gas analyzers.
In spite of the above advantages, cone-like arrays have not been used extensively, mainly due to the difficulties which are involved in their production. Various techniques have been used to fabricate the cone-like structures for decades. Beckey et al. (J. Phys. E. 12, p72, 1979) have used a two-step method to grow dense arrays of metallic needle crystals on a cathode of a vacuum diode. However, this method involves a fairly complicated process, and a very limited number of metals can be fabricated this way. Whitton et al. (Appl. Surf. Sci. 1, p408, 1978) have reported copper cone formation after a bombardment dose of 1019 Ar ions/cm2 during chemical vapor deposition (CVD) growth. This method is fairly effective for most of the metals, however, materials such as silicon and germanium can not be fabricated by this method. Several other growth-induced cone formations have also been reported, including hot filament CVD growth (Chen et al., J. Crystal. Growth, 210, p527, 2000), and Vapor-Liquid-Solid (VLS) technique, in which SiCl4 and H2 are used as the vapor phase (Kiselev et al., Micron 28, p21, 1997). These methods are, however, only effective for one or two types of materials.
Ion beam techniques are widely employed for the purpose of surface texturing. Various materials systems have been investigated (Hudson et al., J. Vac. Sci. Technol., Vol 14, p286, 1977; Floro et al., J. Vac. Sci. Technol., A(1) 3, p1398, 1983; Fujimoto et al., J. Appl. Phys. 77, p2725, 1994; Okuyama et al., Surf. Sci. 338, pL857, 1995). All of them have been able to fabricate the cone-like structures. However, the fabricating area are usually small, the cone-like arrays are not uniform, and the array density is not high. Seeger et al. (Appl. Phy. Lett. 74, p1627, 1999) have developed a method using rough metal films as plasma etching masks, which is able to produce fine silicon cones or pillar-like structures with controllable array density. Their method, however, involves mask-making and the use of poisonous gas such as SF6, which implies increased cost and environmental problems.
It is the object of the present invention to provide one-step methods for fabricating large area uniform silicon cone arrays with various cone morphologies, cone surface modification for field emission applications, and cone-based nano-structure modifications.
The silicon cone array can be prepared by ion-beam sputtering using an ion source in a high vacuum chamber. Metal catalysts may be provided to enable the cone formation. The substrate and metal catalyst should be arranged in a specific configuration to ensure the uniformity of the as-synthesized cone arrays over a large area. The present invention further provides apparatus for ion-beam synthesis of silicon cone arrays, comprising a high vacuum chamber suitable for ion-beam sputtering, a means for holding a substrate in the chamber and a means for arranging the metal catalyst around the substrate.
The field emission properties of the as-synthesized cone arrays can be improved by several surface modification methods, including acid etching, annealing and low work function metal coating.
Silicon and silicon oxide nanowires can be grown from the tips of individual cones. The nanowires on the tips of the cones can be prepared using a hot filament chemical vapor deposition chamber. Argon is used as a protective gas, and hydrogen is used as reductive gas in order to achieve the silicon nanowires. The present invention provides apparatus for nanowire growth, comprising a chemical vapor deposition chamber, means for holding a substrate in the chamber, and means for supporting one or more filaments in the chamber. The deposition time is strictly controlled to enable one nanowire-one cone tip relationship.